Glikmanius is a species of Late Mississippian age shark (~330 myo). Its fossil is found within the limestone ceiling of a cave passage within Mammoth Cave National Park. Sharks fossils other than teeth are exceptionally rare. Lacking hard skeletal structures, sharks have cartilaginous skeletons, and do not easily fossilize. This fossil preserves a portion of the shark’s head, including portions of the upper and lower jaws and even gill structure. In life, this specimen was about 75 cm (2-1/2 ft) in length.
Read more about the fossil sharks here